Siren unit

ABSTRACT

A siren unit including control means for controlling activation of a siren loudspeaker (8) at a selected frequency based on a frequency control signal, and input means for providing the frequency control signal to the control means. The input means further provides a volume control signal and a duration control signal. The siren loudspeaker (8) is activated in response to a trigger signal received by the input means which may be a plurality of predetermined data bits. The siren loudspeaker (8) can be activated on the basis of a data communications packet received and provided by the input means. The siren loudspeaker (8) is also activated when the data communications packet is not received.

BACKGROUND OF THE INVENTION

The present invention relates to a siren or alarm unit, and inparticular a siren or alarm unit which can be used with vehicle securitysystems.

Vehicle security systems may include sirens which are activated by thesystem when an alarm condition occurs, such as unauthorised or forcedentry to a vehicle. The units which drive the siren are normallyconfigured to generate a predetermined siren signal of a set volume andduration. The frequency of activation of the siren may also be set, andin some instances the frequency is varied during the period ofactivation in accordance with a set frequency or sound pattern.

Rigid setting of the frequency, volume and duration is disadvantageousas regional authorities have established different detailedspecifications which place controls of the frequency, volume andduration for use of vehicle sirens. For example, an authority mayspecify maximum and minimum frequencies which can be used, or limits onthe predominate frequency, for any vehicle siren signals generated. Thismeans that different siren units have to be manufactured for eachregion, country or state depending on the authority's requirements.These requirements or regulations then need to be taken into account toensure the correct siren unit is installed in a vehicle, and attentionpaid to the likely region, country or state in which the vehicle will beused. This still, of course, does not overcome the problem of a vehicleowner breaching a local authority's regulations when the vehicle's sirenis activated in another region, country or state from that which it isnormally driven in.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a siren unitincluding control means for controlling activation of a siren soundemitting means at a selected frequency based on a coded frequencycontrol signal; and input means for receiving said coded frequencycontrol signal from a vehicle security control unit and providing it tosaid control means.

The present invention also provides an alarm unit including:

control means for controlling activation of loudspeaker means inresponse to an alarm signal and a sound control signal; and

input means for providing said sound control signal and said alarmsignal to said control means;

said control means storing a predetermined set of sound signals, wherebysaid control means selects one of said sound signals in response to saidsound control signal and, when said alarm signal is received, causessaid loudspeaker means to generate sound in accordance with the selectedsound signal.

The present invention further provides an alarm unit including: controlmeans for controlling activation of loudspeaker means in response to adata communications packet; said control means being connected to avehicle security control unit; input means for receiving said datacommunications packet from the vehicle security control unit andproviding it to said control means; said control means storing apredetermined set of sound signals, whereby said control means selectsone of said sound signals in response to sound control informationcontained in said data communications packet and causes said loudspeakermeans to generate sound in accordance with the selected sound signalwhen an alarm condition is indicated, whereby an alarm condition isindicated when said data communications packet is not received by saidinput means.

The present invention also provides a siren unit including: controlmeans for controlling activation of a siren sound emitting means basedon a data communications packet, said data communications packetcontaining information about the frequency pattern of a sound to beemitted; and input means for receiving said data communications packetfrom a vehicle security control unit and providing it to said controlmeans.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention is hereinafterdescribed, by way of example only, with reference to the accompanyingdrawings, wherein:

FIG. 1 is a block diagram of a preferred embodiment of a siren unit;

FIGS. 2-2C are circuit diagrams of the siren unit of FIG. 1;

FIG. 3 is a schematic diagram of voltage pulses used to control an audiodriver of the siren unit;

FIG. 4 is a diagram of frequency patterns for controlling the frequencyof activation of the audio driver;

FIG. 5 is a diagram of trigger signals for the siren unit;

FIG. 6 is a diagram of the format of a serial communications packet forthe siren unit;

FIG. 7 is a diagram illustrating high and low bits of serial data of thecommunications packet;

FIG. 8 is a block diagram of a further preferred embodiment of a sirenunit; and

FIGS. 9-9C are circuit diagrams of the siren unit of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A siren unit 2, as shown in FIGS. 1 and 2, includes a central processingunit (CPU) 4, an audio driver 6 and a siren loudspeaker 8. The sirenloudspeaker 8 is designed for emitting siren sounds in the frequencyrange 2500 to 3500 Hz but can also be used to emit sounds of otheraudible frequencies. The CPU 4 is configured so as to provide activationsignals to the audio driver 6 so the driver 6 can drive the siren 8, inresponse to a trigger signal received on a control input 10. The controlinput 10 is connected to the CPU by a serial interface 12. The controlinput 10 may be connected to a vehicle security control unit which isable to monitor the vehicle, determine when an alarm condition occurs,and issue the appropriate trigger signal.

The siren unit 2 may be powered by a vehicle battery connected to abattery input 13, the vehicle battery voltage then being regulated by avoltage regulator circuit 14 so as to provide a supply voltage V_(CC)for the CPU 4 and other parts of the unit 2. The siren unit 2 has abackup battery 16 which is used to power the unit and provide the supplyvoltage V_(CC) in the event that supply from the vehicle battery isdisconnected or cut. The backup battery 16 is charged by a batterybackup charger 18 which includes a trickle charging circuit 19 and afast charging circuit 21.

A power cut sensor circuit 20 is included which is able to detect whenthe power supply from the vehicle battery is disconnected and therebycause the CPU 4 to activate the siren 8. The power cut sensor 20 is ableto discriminate between slow discharge of the supply provided on thebattery input 13 and disconnection of the supply. A key switch 22 isprovided to enable the siren unit 2 to be switched on and off as desiredby the possessor of a key for the switch 22. Opening of the key switch22 disables the audio driver 6. This allows the battery to bedisconnected during maintenance or service of the vehicle or siren unit2.

A backup battery monitor circuit 24 is included which is able to monitorthe state of the backup battery and report to the CPU 4. This enablesthe siren unit 2 to issue a signal indicating the voltage which can beprovided by the backup battery 16 is not sufficient. The siren unit 2also includes an ignition interface 26 which has a ignition input 28 forconnection to the ignition line of a vehicle. The ignition interface 26is able to detect when ignition power has been enabled in the vehicleand report to the CPU 4. This allows the CPU 4 to selectively enable thetrickle charging circuit 19 or the fast charging circuit 21 depending onwhether ignition power is being provided to the motor of the vehicle.

The audio driver 6 comprises a H-bridge transistor circuit, as shown inFIG. 2, which is activated by two voltage pulse signals of oppositepolarity generated by the CPU 4 on respective inputs of transistors 30and 32. The H-bridge 6 is able to drive the inputs of the siren 8between a high and low voltage range corresponding to the voltage of thevehicle battery as the transistors 30 and 32 are sequentially activatedin response to the opposite pulses provided on the inputs of thetransistors 30 and 32. The volume of the signal generated by the siren 8can therefore be controlled by varying the width of the voltage pulsesused to activate the audio driver 6. FIG. 3 shows a high volume voltagepulse signal 34 which may be input on one of the transistors 30 or 32and a low volume voltage pulse signal 36. The signal input on the othertransistor 32 or 30 would have the same shape and synchronisation but beof the opposite polarity. The duration or period of activation of thesiren corresponds to the time period for which the voltage pulse signalsare provided to the audio driver 6.

The frequency of activation of the siren 8 during the selected durationmay also be controlled by varying the frequency of the voltage pulses34, 36, i.e., by varying the time period between successive high or lowpulses in the activation signals for the driver 6. The frequency can becontinuously varied in accordance with a frequency or sound patternwhich is stored in the CPU 4. The CPU 4 may store a number of soundpatterns which are chosen so as to meet frequency restrictions which maybe imposed by regulatory authorities for a region, state or country.Three examples of sound patterns which may be used and stored in the CPU4 are shown in FIG. 4. The first sound pattern 38 simply involvesvarying the frequency of the activation signals periodically between ahigh and low frequency whereas the second and third patterns 40 and 42involve a more gradual variation of the frequency of the activationsignals to a peak frequency. The sound patterns 30 to 42 will cause thesiren 8 to generate distinctive siren signals.

The frequency pattern, volume and duration which the CPU 4 uses whenactivating the audio driver 6 in response to a trigger signal isdetermined by either signals received on the serial interface 12 fromthe vehicle security control unit or the state of a set of nine "dip"switches 44 connected to the CPU 4. Table 1 below specifies theinstruction conveyed to the CPU 4 for each state of the switches 44.

                  TABLE 1                                                         ______________________________________                                        Switch              Switch                                                    No.   Description   Status    Configuration                                   ______________________________________                                        1     Trigger Input/                                                                              On        Trigger Input                                         Serial Interface                                                                            Off       Serial Interface                                2     Active High Trigger/                                                                        On        Active High Trigger                                   Active Low Trigger                                                                          Off       Active Low Trigger                              3     Power Cut Detect                                                                            On        Power Cut Detect Enable                               Enable/Disable                                                                              Off       Power Cut Detect                                                              Disable                                         4 & 5 Alarm Duration                                                                              bit4   bit5                                                                   Off    Off  1 Minute Alarm Duration                                           Off    On   2 Minute Alarm Duration                                           On     Off  3 Minute Alarm Duration                                           On     On   4 Minute Alarm Duration                       6 & 7 Alarm Volume  bit6   bit7                                                                   Off    Off  105dBA/1m                                                         Off    On   110dBA/1m                                                         On     Off  115dBA/1m                                                         On     On   120dBA/1m                                     8 & 9 Sound Type    bit8   bit9                                                                   Off    Off  Sound Pattern 1                                                   Off    On   Sound Pattern 2                                                   On     Off  Sound Pattern 3                                                   On     On   Sound Pattern 4                               ______________________________________                                    

The state of the first switch indicates whether the siren unit 2 will betriggered by the contents of a control data packet received by theserial interface 12 or a trigger signal 50 or 52 on the control input10, as shown in FIG. 5. If serial interface control is disabled byswitching off switch 1 then switch 2 will indicate whether the CPU 4needs to look for a high trigger signal 50 to activate the siren 8 or alow trigger signal 52 to activate the siren 8. The ability to triggerthe siren 8 on the basis of a control packet received by the serialinterface 12 or high or low trigger signals 50 or 52 is provided toensure the siren unit 2 is compatible with different types of triggeringused by vehicle security control units. A vehicle security control unitdeveloped by the applicant is able to generate appropriate controlpackets to trigger the siren 8 by the serial interface 12.

The siren unit 2 is configured so that if the control input 10 isdisconnected or cut, whilst the unit 2 is enabled, the unit 2 willtrigger the siren 8. If serial interface control is disabled, theinterface 12 will be pulled high or low, corresponding to the level ofthe trigger signal 50 or 52, when the control input 10 is disconnectedso as to trigger the siren 8. This introduces a further level ofsecurity to the unit 2 which can only be defeated if the control input10 is disconnected and simultaneously maintained low or high so as toprevent the interface 12 detecting a voltage difference and passing atrigger signal to the CPU 4. If serial interface control is enabled, thesecurity of the siren unit 2 is enhanced even further. To prevent thesiren 8 from triggering, the CPU 4 must continuously or regularlyreceive a control packet from the vehicle security control unit whichprevents the siren 8 from being activated. Whilst a thief may be able toprovide the correct voltage level to the interface 12 when disconnectingthe input 10, and it is highly unlikely that a thief would be able toprovide the correct serial code of the control packet, which asdescribed below, may comprise 24 bits, to prevent the siren 8 beingactivated on disconnection.

When the trigger input looks to the high or low trigger signals 50 or52, the duration, volume and sound pattern used by the CPU 4 isdetermined by the state of the fourth to ninth switches 44 as specifiedin Table 1. The siren 8 will remain activated during the period of thehigh or low trigger signal 50 or 52 unless the chosen alarm durationexpires first.

Switching the first switch of the dip switches 44 to "off" enables theCPU 4 and the siren unit 2 to be controlled by 24 bit serial controlpackets 54, as shown in FIG. 6. The packet format 54 is used by the CPU4 to receive instructions from a vehicle security control unit on thecontrol input 10 and also to send information to the vehicle securitycontrol unit. The CPU 4 receives a control packet 54 from the vehiclesecurity control unit every one second and if a valid control packet 54is not received the CPU 4 activates the siren 8. The informationrepresented by each bit of the serial data packet

    ______________________________________                                        Bit No.                                                                             Description    Bit Status                                                                              Configuration                                  ______________________________________                                        0..3  Address Field  0001      Siren Address                                  4..7  Spare          --        --                                              8    Spare          --        --                                                                  --        --                                              9    Power Cut Detect                                                                             1         Power Cut Detect                                     Enable/Disable           Enable                                                              0         Power Cut Detect                                                              Disable                                        10, 11                                                                              Alarm Duration bit10  bit11                                                                  0      0    1 Minute Alarm                                                                Duration                                                          0      1    2 Minute Alarm                                                                Duration                                                          1      0    3 Minute Alarm                                                                Duration                                                          1      1    4 Minute Alarm                                                                Duration                                     12, 13                                                                              Alarm Volume   bit12  bit13                                                                  0      0    105dBA/1m                                                         0      1    110dBA/1m                                                         1      0    115dBA/1m                                                         1      1    120dBA/1m                                    14, 15                                                                              Sound Type     bit14  bit15                                                                  0      0    Sound Pattern 1                                                   0      1    Sound Pattern 2                                                   1      0    Sound Pattern 3                                                   1      1    Sound Pattern 4                              16    Trigger Alarm/Shut Off                                                                       1         Trigger Alarm                                        Alarm          0         Shut Off Alarm                                 17    Reset Alarm    1         Reset Alarm Timer                                    Timer/Continue Timer                                                                         0         Continue Alarm                                                                Timer                                          18    Backup Battery Status                                                                        1         Backup Battery                                 *Note 1                                                                             Request                  Request                                                             0         No Request                                     18    Backup Battery Status                                                                        1         Backup Battery Fiat                            *Note 2              0         Backup Battery OK                              19    Backup Charging Status                                                                       1         Fast Charging Backup                                                0         Trickle Charging                                                              Backup                                         20    Siren Status   1         Alarm Sounding                                                      0         Siren Quiet                                    21    Vehicle Battery                                                                              1         Battery Connected                                    Connection Status                                                                            0         Battery Disconnected                           22    Ignition Status                                                                              1         Ignition On                                                         0         Ignition Off                                   23    Key Switch Status                                                                            1         Key Switch On                                                       0         Key Switch Off                                 ______________________________________                                         *Note 1: Data received by the siren.                                          *Note 2: Date transmitted by the siren.                                  

Bits 10 to 15 are used for alarm duration, volume and sound patternselection, as discussed previously. This allows the vehicle securitycontrol unit to select between four siren frequency or sound patterns,four volumes and four siren durations as desired. Of course, furtheroptions can be simply provided by increasing the number of bitsavailable for siren configuration and storage capacity of the CPU 4, ifnecessary. The ultimate selection made by the vehicle security controlunit can be based on a number of factors. The vehicle security controlunit when installed may include a vehicle type, code or country codewhich indicates where the vehicle is most likely to be driven and thecontrol unit can then make a siren configuration selection on the basisof that code. Alternatively, the vehicle may include a positioningsystem, such as a Global Positioning System (GPS), and on the basis ofinformation provided by the positioning system, the vehicle securitycontrol unit can instruct the siren unit 2 to select a siren frequency,duration and volume which is the most suitable for the vehicle's currentposition. Vehicle instrument clusters also tend to include a vehicletype or country code and this code can be provided to the vehicle'ssecurity control unit as a basis for determining the correct sirenconfiguration to be set for the siren unit 2.

The siren configuration data can be updated each time the vehiclesecurity control unit communicates with the siren unit 2 via the serialinterface 12. An address field of 4 bits is provided so the vehiclesecurity control unit can address devices other than the siren unit 2 ona single serial data bus. The data is preferably transferred at 500 baudwhich gives the packets 54 a period of 48 ms. The length of each bit istherefore 2 ms and pulse width modulation is used to encode each bitsuch that the duty cycle determines whether the bit is a low bit 56 or ahigh bit 58, as shown in FIG. 7.

The CPU's 4 response to detection of power disconnection by the powercut sensor 20 can be enabled or disabled, as shown in Tables 1 and 2.The response may be disabled to allow the vehicle battery to bedisconnected during servicing or maintenance of a vehicle. When enabled,if disconnection of power supply from the battery input 13 is detectedby the sensor 20 and the siren 8 is currently silent, the CPU 4 willthen generate the appropriate activation signals selected on the basisof the current siren configuration so as to cause the audio driver 6 tosound the siren 8. Power is supplied to the CPU 4 and the othercomponents by the backup battery 16, which may be a 7.2 volt nickelcadmium battery. The backup battery 16 will also continue to providepower when supply to the battery input 13 is disconnected and the siren8 is currently activated so as to ensure the siren continues to operateat the configured volume, frequency and duration.

False triggering of the siren 8 by the power cut sensor 20 by flatvehicle batteries is prevented by monitoring the rate of decrease in thesupply voltage on the input 13. The power cut sensor 20 provides atrigger signal for the CPU 4 if the voltage on the battery input 13decreases from 8 to 6 volts in less than one second. If voltage supplydecreases from 8 to 6 volts in greater than one second, i.e., as wouldbe the case for slow battery discharge, the power sensor 20 does notprovide a trigger signal for the CPU 4. This enables the power cutsensor 20 to discriminate between slow battery discharge andunauthorised disconnection of the battery.

The trickle charge circuit 19 provides a trickle charge current to thebackup battery 16 so as to charge the backup battery 16 withoutdischarging the vehicle battery. The trickle charge current does notovercharge the backup battery 16 and is safe for use for indefinitecharging periods. A faster charging current is provided by the fastcharge circuit 21 when the CPU 4 receives a signal from the ignitioninterface 26 indicating that the ignition input 28 is high. The fastcharging circuit 21 is disabled, to prevent overcharging, when thebackup battery monitor circuit 24 detects that the backup batteryvoltage is greater than 9 volts. Nickel cadmium batteries are notsuitable for charging at very high and very low temperatures, andtherefore a temperature sensor may be included in the siren unit 2 whichwould cause the fast charging circuit 21 to be disabled when the sensortemperature is too high or too low. Battery life can thereby beprolonged, particularly in regions which regularly experience very highor very low temperatures.

The backup battery monitor circuit 24 is able to detect when the backupbattery is flat, i.e. at a voltage less than 6 volts. The vehiclesecurity control unit can also request, using bit 18 of thecommunications packet 54, a report on the backup battery status, and theCPU 4 then performs a flat battery test using the backup battery monitor24. A load is applied to the backup battery 16 for 1 s while the batteryvoltage is checked using the monitor circuit 24. If the backup voltageexceeds 6 volts then the CPU 4 reports, using bit 18, that the backupbattery is satisfactory, otherwise the CPU reports that it is flat. Onreceiving the flat backup battery status, the vehicle security controlunit may generate a warning signal to the vehicle driver, for example,by causing an LED on the instrument panel of the vehicle to flash. TheCPU 4 is also able to report, using the serial interface 12, otherstatus information on the state of its components and inputs using bits19 to 23 as shown in Table 2.

An alternative siren unit 100, as shown in FIG. 8, is essentially thesame as the siren unit 2 described previously but includes additionalfeatures and omits other features of the siren unit 2 as discussedbelow.

The key switch 22 is effectively replaced by a backup battery enablecircuit 102 which is controlled by the CPU 4. The enable circuit 102 isconnected between the backup battery 16 and the components which thebattery 16 supplies power to, and is enabled when the siren unit 100receives power from the vehicle battery on the line 13. In response toinstructions, received via the control input 10, that the siren unit 100is to be disabled, such as for maintenance, the CPU 4 issues a signal todisable the battery backup enable circuit 102 so that the backup battery16 can not act to power the unit 100. This allows the unit 100 to beserviced or maintained without the siren 8 sounding on disconnection ofthe power supply to the battery input 13. If serial interface control isenabled, one of the bits in the control packet can be used to instructthe CPU 4 to disable the battery backup enable 102. If serial interfacecontrol is disabled, the vehicle security control unit could pass acoded series of high and low voltage signals on the control input 10 tothe CPU 4. For example, a user may enter a four digit code which isacknowledged by the vehicle security control unit by moving an ignitionkey into and out of the ignition position on an ignition lock of avehicle in a predetermined sequence.

If serial interface control is enabled, the siren unit 100 may also omitthe ignition interface 26 as the vehicle's security control unit can beused to monitor the state of the ignition line 28 and report using thecontrol packet to the CPU 4.

The siren unit 100 further includes a horn interface circuit 104connected to the CPU 4 and which receives activation signals on a hornswitch input line 106 from the horn switch of a vehicle. With the horninterface 104 and the horn switch input 106 connected, a vehicleincluding the siren unit 100 does not require a horn. On depressing thehorn switch, an activation signal is provided on the input 106 to theinterface 104 which instructs the CPU 4 to access an appropriate soundpattern for a vehicle horn. The sound pattern is used by the CPU 4 tocontrol the audio driver 6 so as to cause the siren loudspeaker 8 toemit the sound of a vehicle horn, which is normally at a frequency ofabout 450 Hz. Inclusion of the horn interface 104 and the input 106,with the control software of the CPU 4 altered accordingly, allows amanufacturer to save on the cost of a vehicle horn and relay. The sirenunit 100 will sound the horn sound in response to activation of the hornswitch, regardless of the state of the remaining components and alarmfeatures of the siren unit 100, provided the vehicle battery powersupply 13 is connected.

Further variations which the siren unit 100 includes is the use of asupply filter 110 between the vehicle battery input 13 and the voltageregulator 14 so as to protect the regulator from voltage transients andpeaks on the battery input 13. The unit 100 also includes a non-volatilesemiconductor memory 112 which may be used instead of the "dip" switches44 to include bits of data which can provide the same instructions tothe CPU 4 as the state of the switches, in accordance with the controlallocations set out in Table 1.

The backup battery 16 is connected to two junction points 120, as shownin FIG. 9, with the backup battery enable circuit 102 including threetransistors and respective bias resistors configured to disable thesupply of input power to the voltage regulator 14 on receipt ofinstructions from the CPU 4. The trickle charge circuit 122 is locatedbetween the junction points 120 and the battery input 13. The fastcharging circuit 124 includes a transistor and resistor arrangementshown next to the backup battery monitor circuit 24 and the horninterface circuit 104 in FIG. 9. The voltage regulator 14 and the memory112 are standard products, such as that produced by S.G.S. Thompsonswith product numbers L49490N and ST93C06CM3, respectively. The CPU 4 isa standard microprocessor, such as the Motorola MC68HC05P6microprocessor, which obtains its clocking signals from a resonator 126.The circuit diagram includes a temperature sensor circuit 128 whichadjusts the level of the monitored supply voltage V_(CC) that is passedto the CPU 4. The monitored voltage level is adjusted to prevent the CPU4 from instructing the fast charging circuit 124 to be activated whenthe sensor temperature is very high or very low, for example above 65°C. and below 0° C.

The siren unit 2 is particularly advantageous as it can be connected toa number of vehicle security control units and can be simply configuredusing the dip switches 44 or serial interface 12 so as to emit a sirensignal of a selected frequency, duration and volume. The siren unit 2also provides a facility for complete configuration control by aconnected vehicle security control unit to which the status of the sirenunit 2 can be reported as requested. The siren configuration can bebased on country or car codes included in the vehicle security controlunit or provided to it by other electronic components of the vehicle.Also if the vehicle includes a positioning system, then sirenconfiguration can be selected based on vehicle position thereby allowingcontinual compliance with siren regulations set by local authorities.The siren unit is also able to detect unauthorised disconnection anddiscriminate between power disconnection and slow power supplydeterioration.

The claims defining the invention are as follows:
 1. A siren unitincluding:control means for controlling activation of a siren soundemitting means at a selected frequency based on a coded frequencycontrol signal; input means for receiving said coded frequency controlsignal from a vehicle security control unit and providing it to saidcontrol means; and, power sensor means which detects disconnection of apower supply to said unit and discriminates between said disconnectionand gradual deterioration of said power supply.
 2. A siren unit asclaimed in claim 1, wherein said control means stores a predeterminedset of frequency patterns and selects one of said patterns on the basisof said frequency control signal, the selected pattern thereby selectingsaid frequency of activation of said siren sound emitting means.
 3. Asiren unit as claimed in claim 2, wherein said selected frequency isvaried over time by varying the frequency of the activation signal forsaid siren sound emitting means according to said selected pattern.
 4. Asiren unit as claimed in claim 1, wherein said input means furtherprovides a volume control signal and a duration control signal, and saidcontrol means controls activation of said siren sound emitting means ata selected volume for a selected duration based on said volume andduration control signals.
 5. A siren unit as claimed in claim 1,including battery means for supplying power to said unit ondisconnection of said power supply, said power sensor means causing saidcontrol means to activate said siren sound emitting device on detectingsaid disconnection.
 6. A siren unit as claimed in claim 5, includingbattery charging means for charging said battery means with a tricklecurrent when an ignition power line is in one state and charging saidbattery means with a relatively fast charging current when said ignitionpower line is in another state.
 7. A siren unit as claimed in claim 5,wherein said control means is adapted to perform a diagnostic test on atleast one part of said unit and issue a reporting signal to report theresults of said test.
 8. A siren unit as claimed in claim 7, includingmonitoring means for performing said diagnostic test on said batterymeans and causing said reporting signal to indicate the power of saidbattery means is below a predetermined level when said results of saidtest determine said power is below said predetermined level.
 9. A sirenunit as claimed in claim 8, wherein said control means conducts saidtests in response to receiving a diagnostic test request signal.
 10. Asiren unit as claimed in claim 1, wherein said siren sound emittingmeans is activated in response to a trigger signal being received bysaid input means.
 11. A siren unit as claimed in claim 10, wherein saidtrigger signal is a plurality of predetermined data bits.
 12. A sirenunit as claimed in claim 10, wherein, in use, at least one commandsignal is provided to said input means and said trigger signal is anysignal received by said input means in the absence of said at least onecommand signal.
 13. A siren unit as claimed in claim 1, wherein saidinput means is connected to a vehicle security unit and said powersupply comprises a vehicle battery.
 14. A siren unit as claimed in claim5, wherein said control means includes a microprocessor and an audiodriver and said input means includes a data interface.
 15. A siren unitas claimed in claim 1, wherein said control means includes amicroprocessor and an audio driver and said input means includes atrigger input and a set of configuration switches.
 16. A siren unit asclaimed in claim 1, wherein said control means includes a microprocessorand an audio driver and said input means includes a trigger input andmemory means storing configuration data.
 17. A siren unit as claimed inclaim 4, wherein said siren sound emitting means is activated inresponse to a trigger signal being received by said input means, andsaid siren sound emitting means ceases to be activated on expiry of saidselected duration or upon the absence of said trigger signal at saidinput means, whichever occurs first.
 18. A siren unit including:controlmeans for controlling activation of a siren sound emitting means at aselected frequency based on a coded frequency control signal; inputmeans for receiving said coded frequency control signal from a vehiclesecurity control unit and providing it to said control means; and,wherein said input means is connected to a vehicle security unit whichprovides said frequency control signal on the basis of positionalinformation concerning a vehicle in which the units are installed.
 19. Asiren unit including:control means for controlling activation of a sirensound emitting means at a selected frequency based on a coded frequencycontrol signal; input means for receiving said coded frequency controlsignal from a vehicle security control unit and providing it to saidcontrol means; and, wherein said input means is connected to a vehiclesecurity unit which provides said frequency control signal on the basisof regional information associated with a vehicle in which the units areinstalled.
 20. A siren unit as claimed in claim 1, wherein said inputmeans includes horn means for providing said frequency control signal tosaid control means so as to cause said control means to activate saidsiren sound emitting means at a selected frequency which corresponds tothe frequency of a vehicle horn.
 21. A siren unit as claimed in claim 5,including disabling means for preventing activation of said siren soundemitting device on said disconnection of said power supply.
 22. A sirenunit including:control means for controlling activation of a siren soundemitting means based on a data communications packet, said datacommunications packet containing information about the frequency patternof a sound to be emitted; input means for receiving said datacommunications packet from a vehicle security control unit and providingit to said control means; and wherein said control means causesactivation of said sound emitting means when said data communicationspacket is not received by said input means.
 23. A siren unitincluding:control means for controlling activation of a siren soundemitting means at a selected frequency based on a coded frequencycontrol signal; input means for receiving said coded frequency controlsignal from a vehicle security control unit and providing it to saidcontrol means; wherein said siren sound emitting means is activated inresponse to a trigger signal being received by said input means; andwherein, in use, at least one command signal is provided to said inputmeans and said trigger signal is any signal received by said input meansin the absence of said at least one command signal.